This invention relates to a method and apparatus to improve the positioning accuracy of a tracking arm as it follows a selected position on the surface of a rotating member, and more particularly, this invention relates to a method and apparatus for use wherein the rotating member is subject to the phenomenon of run-out.
In data storage apparatus of the type using rotating disc or a stack of rotating discs as the recording medium, data is usually recorded in a plurality of concentric data tracks on the surface of the disc. The tracks are closely spaced, for example, at 480 tracks per inch, and the data recorded at densities in excess of 9,000 bits per inch. In order to access the recorded data, it is necessary to move a record and playback head (or transducer) to the selected track in which the desired information is stored, and maintain the head precisely over the center of the track with a minimum displacement error in the presence of any possible displacement error for the whole time the information is being read or being written.
As track density increases, it becomes more imperative that the record and playback head, or transducer, be accurately maintained over the selected track in which the desired information is either being read or being written. One factor limiting the accuracy of the tracking position of the transducer over the selected position is the mechanical tolerance in the head positioning motor and the support system. In addition, in some recording discs, which are removable, the abutting surfaces of the disc and the disc drive spindle may cause additional errors. One resultant error from the foregoing mechanical tolerances is the phenomenon of run-out. Run-out is caused by eccentricity of the disc or of the spindle or both. Run-out is the result of the eccentricity or the wobbling of the disc, wherein the center of the disc rotation does not coincide with the center of the data track or of the written material. Run-out may be observed even in common day experience by watching the behavior of a tracking arm in a phonograph record player wherein the arm moves to and fro in a sideways motion as it follows the tracks of a warped phonograph.
One technique taught by the prior act of eliminating run-out is disclosed in U.S. Pat. No. 3,808,486. Since the disc spins at a constant frequency, run-out occurs at the same known constant frequency. In that patent, the amplifier supplying the power to the actuator which moves the tracking arm has a non-linear response, whereby at the run-out frequency, there is an increased amount of gain from the amplifier to the actuator which moves the transducer. In this manner, the patent teaches that the actuator would be moved an additional amount at the run-out frequency in order to be in phase with the run-out to keep accurate tracking over the tracks. Although this is a viable solution to the problem of run-out, the solution taught by the patent is not desirable for a number of reasons. First, the response time of this servo system is slow. Because both the frequency and the gain have a distribution of values, or a sigma, the increase in gain at the run-out frequency is a broad band. Thus the response factor is slow which requires several revolutions of the disc for the gain increase of this technique to build up. Next, since the teaching of that patent is to change the frequency and gain response of the power amplifier that supplies the current to the actuator, the electronic circuitry to implement that teaching is necessarily complicated and is expensive.